The main field, secular variation, and secular acceleration spectra, extracted at yearly intervals from the parent model, are shown in Figure 9.
The magnetic field at the Earth's surface is mainly composed of three contributions with different origins: core-derived main field, temporal variation relating to ionospheric and magnetospheric sources, and an anomaly due to lithospheric sources (e.g., Sabaka et al. 2002).
We derive an additional built-in model of the Earth's main and lithospheric internal field up to SH degree 60, the main field secular variation and acceleration up to SH degrees 10 and 8, respectively, and a simple model for the magnetospheric field to SH degree 1 scaled with the hourly Dst index [details about this relatively standard parameterization may be found in Thébault et al. (2010a)].
The last two candidate models rely on statistical analyses of the main magnetic field variation over the past centuries and on core flow hypotheses to forecast the field (candidate models A and C).
Spherical cap harmonic functions in particular have been widely applied for modeling main field and secular variations (e.g., Haines 1985; Kotzé 2001; Gaya-Piqué et al. 2006), and lower-order lithospheric components of the Earth's magnetic field (Coles 1985; de Santis et al. 1989; Kovács et al. 2011).
Each month, a new set of matrices relating flow velocity and acceleration to secular variation and secular acceleration are computed based on the main field and secular variation coefficients.
In section 'Main field and secular variation candidates for IGRF 2015', we discuss our candidate models for the 2015 IGRF main field and secular variation, which were both extracted from the same parent model.
For the 2015.0 epoch, NGDC has produced candidate models for both the main field and secular variation.
The plots show the spectra for the main field (top), secular variation (middle), and secular acceleration (bottom).
Results of the statistical comparisons between measured and modelled main field and secular variation components are summarised in Table 1.
